2017 Western Iowa Labor Day Palmer Amaranth Tour

This pigweed had been pulled out of a Georgia field and dropped in the furrow. As often happens, the plant sent out new roots and began growing new foliage and would have produced more seed to fuel future infestations. Photo: Rome Ethredge.

For the past four years (maybe five) I’ve visited the first three known infestations of Palmer amaranth in western Iowa to track the status of the populations in these fields. Following is a brief summary of my visits to Harrison, Fremont and Page Counties on September 3.

First stop was ground zero for Palmer in Iowa, a field near Modale in Harrison County. I encountered the first Palmer plant in the ditch about a 1/3 of a mile down the road from what is believed to be the source field.

The field was in corn for the first time since I’ve been visiting it, so it was difficult to compare the infestation severity to previous years. However, there seemed to be fewer plants along the field edges than in earlier years. Unfortunately, there were a few good-sized plants in a ‘waste-area’ in a corner of the field (Figure 1).

An area in perennial grass across the road that had a healthy Palmer infestation from 201 to 2015 was Palmer-free.

Figure 1. Palmer in waste area with no competition.

The Harrison County field is bordered by a DNR public hunting area on one side. As I scouted that side of the field I came across a tree with hundreds of roosting monarchs that took flight as I drove by. My photography skills failed to catch the scale of the monarchs leaving the tree (Figure 2). I did not find any Palmer in this area.

Figure 2. A few of the monarchs leaving the roost tree due to my presence.

Second stop was Fremont County. This is an interesting site in that the infestation is inside a small town. Palmer was first found in a small (<2 acre) field used for seed show plots, and the field has always has had a low number of escapes (<50 plants).

The benefit of having Palmer classified as a noxious weed was evident at this site. The weed commissioner had contacted the seed company and informed them of the infestation in late August. The hand pulled plants were laying in the grass areas adjacent to the field (Figure 3).

Figure 3. Hand-roguing is needed in new infestations to prevent permanent infestations.

Unfortunately, there was evidence that Palmer has moved from the initial field. A sweet corn patch across the road had a significant infestation of Palmer, and I found a small patch (approximately 200 sq ft) in a ditch about a 1/3 of a mile down the road. I had never noticed Palmer in that area before.

Final stop was Page County – this infestation is in a crop field adjacent to an ag retailer. In previous tours, I’ve found Palmer in both the crop field and the grounds of the retailer. The amount of Palmer in both areas has declined over the course of my surveys. This year I did not find any Palmer in the field, and only a single plant in the retail area.

The Palmer infestation in the field had always primarily been in drainage ways where there was also a healthy waterhemp population. Over the four years the percentage of Palmer in these areas compared to waterhemp has declined.

It has been encouraging to see that the Palmer amaranth populations are not ‘exploding’ in these early infestations. However, it is disappointing that the land managers haven’t adopted a zero threshold.

The Fremont County infestation is concerning because there is evidence that Palmer is spreading. I haven’t observed that at the other locations. Hopefully having Palmer amaranth classified as a noxious weed will provide the incentive for land managers to go the extra mile to stop the spread of Palmer amaranth.